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HOLLISTON — After four new cases of Eastern equine encephalitis were confirmed in horses, state public health officials Thursday raised the risk level for the mosquito-borne virus to its highest point in more communities across Massachusetts.

Granby, Brookfield, Holliston, and Medfield were found to be at critical risk after horses in each town were infected by the virus, which killed one animal and forced the other three to be euthanized, state officials said.

Thirteen other communities, ranging from Walpole to Sturbridge to Belchertown, had their risk level raised to high, the second-highest category, according to the state.

The action came after four human cases of EEE were confirmed in the state this year, including one that led to the death of a woman from Fairhaven.

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As of Thursday, 191 Massachusetts communities have been determined to be at least at moderate risk of the fearsome disease. Of that number, 28 communities are at a critical risk, the highest alert.

The “critical risk” distinction prompts the state to advise those communities to consider canceling or rescheduling outdoor gatherings, including organized sporting events, to avoid peak mosquito hours.

The increased risk prompted several communities to take swift measures to protect the public from the rare, but sometimes fatal, virus. Holliston and Medfield have curbed outdoor activities after 7 p.m. until further notice.

Residents who do go outside are urged to use mosquito repellent, among other measures.

And mosquito spraying by truck was scheduled to start Friday at dusk in both Holliston and Medfield, according to notices posted on each town’s website.

Joseph Popper, a Holliston resident, said his children’s soccer practice at Brentwood Conservation Land was canceled Thursday evening due to the increased risk.

“I still let them play, but we all sprayed [with bug spray] before,” Popper said.

Jim Giammarinaro, owner of Putts & More in Holliston, said he sprayed the mini golf course to guard against the mosquitoes. The course has been busier during the day, he said.

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“I can’t say if it’s because school started, but the [EEE threat] may have pushed business towards the daytime,” Giammarinaro said.

More serious golfers are reluctant to get off the greens, however. At Pinecrest Golf Club, some players were still on the Holliston course at about 8 p.m. Thursday, despite officials encouraging them to be off the course by dusk, a pro shop employee said.

Communities in the state as far north as Methuen, as far south as New Bedford, and as far west as Heath also have the “critical risk” distinction.

The increased warnings come as Massachusetts school districts gear up for the fall sports season.

Preseasons are already underway throughout the state, with the regular season for many high school sports slated to start in the middle of next week. The high school football season kicks off next Friday.

Eric Scott, the athletic director of Medfield High School, said for outside practices and games, students must be off the fields by 7 p.m. because of the EEE risk. Earlier this week, Holliston public schools also announced that all outdoor activities would be finished by 7 p.m. The district said that such an order would be reassessed on Sept. 3, following “the receipt of further information’’ from the state.

EEE infection in humans is extremely rare. Just 28 people in Massachusetts have been diagnosed with the disease since 2004, according to the Centers for Disease Control and Prevention.

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But the virus is particularly deadly — with a 40 percent mortality rate — and those who survive often suffer from some long-term neurological deficit.

Before the most recent spate of EEE cases, Massachusetts hadn’t seen an active case in a human since 2013. The state is now starting a new, intense cycle of EEE activity that will likely persist for two to three years, state officials told the Globe earlier this week.

In response, state agriculture officials have sprayed parts of Bristol, Plymouth, Middlesex, and Worcester counties for mosquitoes, according to a statement from the state’s Department of Public Health.

“Spraying does not eliminate the risk of EEE transmission and the public is asked to continue to follow personal protection practices,” the statement said.

“The peak time for transmission of mosquito-borne illness extends through September here in Massachusetts,” said Dr. Monica Bharel, the DPH’s commissioner.

A horse in Northwood, N.H., also tested positive for EEE Wednesday, the New Hampshire Department of Health and Human Services said in a statement. This was the state’s first horse infected with EEE this year. Northwood’s EEE risk was elevated to high after the finding.

EEE risk levels were elevated from low to moderate in seven towns near Northwood, the department said.

EEE hits horses harder than humans, said Jake Leon, the department’s director of communication. Horses rarely recover from the illness.

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All animals infected with EEE in Massachusetts this year either died from the disease or were euthanized, according to the state. A horse can still contract the EEE if it is vaccinated, although the disease is usually less severe, according to authorities. None of the horses that contracted the disease in the state this year were reported as having been vaccinated, according to the Massachusetts DPH.

“As we head into the Labor Day weekend and the month of September, people should not forget to bring and use an EPA-approved mosquito repellent for any outdoor activities,” said Bharel.

Those who own horses should keep the animals in indoor stalls at night to reduce their risk of exposure to mosquitoes, and speak with their veterinarians about mosquito repellents and vaccinations, state officials said.

Due to an editing error, an earlier version of this story misstated the risk level in the town of Ashby. It is at a low risk level.


Jaclyn Reiss, Hanna Krueger, and Deanna Pan of the Globe staff and correspondents Maria Lovato and Alyssa Lukpat contributed to this report. Danny McDonald can be reached at daniel.mcdonald@globe.com Sofia Saric can be reached at Sofia.Saric@globe.com